Proper concrete pouring is essential and requires qualified individuals and focused concentration. Codes have set specific specifications to ensure correct concrete pouring and prevent segregation. This article presents the guidelines for correct concrete pouring based on ACI 304R-00.
Fig.1: Concrete Pour Placement
Precautions during Concrete Pouring
The ACI 304R-00 provides precautions for pouring concrete. In normal conditions, where the temperature is not greater than 30 degrees, the time between adding water to the dry mixture and pouring the concrete should not exceed 30 minutes. In hot weather conditions, this time decreases to 20 minutes, but it can be extended by using admixtures.
To ensure proper pouring, suitable equipment and techniques should be employed to allow the concrete to drop vertically without any obstructions to the specified locations. The free fall of the concrete should be continuous, and measures should be taken to prevent any separation that may occur when the concrete falls over reinforcement or other embedded objects.
The distance of free fall for the concrete is generally limited to 0.9 to 1.5 meters, although in some cases, it may be restricted to 0.6 meters. It is important to place the concrete at or near its final position to prevent segregation, ensuring that the different components of the mixture remain properly distributed.
Ideally, the concrete should be discharged directly into the forms, avoiding the use of equipment like chutes or hoppers. This can be achieved if the forms are wide and open, allowing for a direct and unobstructed placement of the concrete.
ACI Code Recommendation for Concrete Pouring of Different Reinforced Concrete Elements
Suitable methods of concrete pouring for different reinforced concrete elements, such as columns and walls, slabs, inclined slabs, and curved members, will be discussed in this section. By applying these appropriate techniques correctly, the risk of segregation and honeycomb formation during the concrete pouring process can be prevented.
Pouring of Concrete in Columns and Walls
When dealing with tall structures, it is advisable to pour concrete in layers. The thickness of each layer typically falls between 300mm and 500mm. It is important to note that the time interval between successive layers should not exceed 30 minutes under normal weather conditions and 20 minutes under hot weather conditions. During the pouring process, it is essential to compact each layer thoroughly using vibrators. Figure 2 provides a visual representation of the correct (A) and incorrect (B) methods of pouring concrete into deep forms in layers. Figure B should be avoided as it can result in the concrete striking the form face, leading to segregation.
If the height of these elements is large, then it is recommended to place concrete in layers. The layer thickness ranges from 300mm to 500mm and the time between successive layers should not exceed 30 minutes for normal weather condition and 20 minutes for hot weather condition. Each layer should be compacted properly using vibrators. Figure 2 explains correct (A) and incorrect (B) method of concrete pouring into deep forms in layers. Figure B is not desired since concrete would strike form face and segregation would occur.
Fig.2: Pouring concrete into deep forms in layers, A) Proper way of pouring concrete, B) incorrect method of pouring
When the height of forms is not significant and concrete is poured from the top of the forms, the ACI code suggests using a light hopper to discharge the concrete. The concrete is then placed into its final location.
Fig.3: Recommended way of pouring concrete into narrow forms
Insufficient and uneven pouring of concrete, as depicted in Figure 4, can result in negative outcomes. When concrete strikes the form with force, steady pouring is not achieved. This can lead to separation within the concrete mixture, causing the formation of honeycomb voids at the bottom.
Fig.4: Improper pouring of concrete into narrow forms such as walls and columns
Pouring of Concrete in Slabs
ACI Code recommends placing concrete into the face of existing concrete, as depicted in Figure 5, rather than away from the face, as shown in Figure 6.
Fig.5: Pouring concrete into the face of concrete in place as recommends by ACI 304R-00
Fig.6: Incorrect pouring of concrete which should be prevented during slab construction
Pouring of Concrete for Inclined Slabs
Pouring concrete on inclined surfaces is typically more challenging than on horizontal surfaces, requiring additional effort to prevent segregation. To mitigate the risk of concrete separation and ensure that it remains in place on the slope, the ACI 304R-00 recommends the use of baffles and drops at the end of the chute. These measures help maintain the integrity of the concrete and prevent it from segregating or flowing unevenly.
Fig.7: Pouring concrete on slope slab using baffle to prevent concrete separation
Without the utilization of the mentioned technique, concrete is prone to experiencing separation, resulting in the settling of large-sized aggregates at the bottom. This phenomenon can be observed in Figure 8.
Fig.8: Improper pouring of concrete, large size aggregates settled at the bottom
Pouring of Concrete for Curved Members
Curved members, such as beams, columns, walls, or slabs, can be found in various structural applications. When it comes to concrete construction, the ACI 304R-00 provides recommendations for effectively pouring concrete in layers. This pouring technique is illustrated in Figure 9 of the document.
Fig.9: A) Correct method of concrete pouring, B) Incorrect method of concrete pouring
Concrete Pouring Using Pumps
Pumps have become the preferred choice for pouring concrete in modern times, replacing traditional equipment. To prevent issues like segregation and other undesirable effects, it is recommended to extend the hose of the pumps as close as possible to the final position of the concrete. Figure 10 and Figure 11 illustrate the recommended method of placing concrete using pumps for both straight and curved members.
Fig.10: Extending pump hose near final location of concrete
Fig.11: Extending pump hose close to the final position of concrete
References:
ACI Committee 304 (2009) published a guide titled “Guide for Measuring, Mixing, Transporting, and Placing Concrete” in Michigan, which provides information on concrete handling processes. This publication is from the American Concrete Institute.
The ASCC (American Society of Concrete Contractors), based in St. Louis, has a publication on the topic of “Free Fall of Concrete,” although no specific date is mentioned.
In 2018, Gates Concrete Forming Systems released a publication called “Gates Concrete Forms,” which can be accessed as of February 10, 2018.
A book by Mohamed A. El-Reedy (2009) titled “Advance Materials and Techniques for Reinforced Concrete Structure” discusses innovative approaches and materials for reinforced concrete structures. The book is published by CRC Press in Boca Raton.
Morgana Matus’s work can be found on the website “Inhabitat,” which was accessed on February 10, 2018. No further details are provided about the specific context or content of the information on the website.